FIG. 1 illustrates a conventional implantable cardioverter-defibrillator (ICD). An electrode structure 40 has been passed thought the subclavian vein 11 into the heart 1. Therapy electrodes 42 are used to deliver energy to shock the heart if and when abnormal cardiac activity is detected. Sensing and/or pacing electrodes 44 are used to monitor cardiac activity and/or pace the heart if necessary. The housing 10 contains the conventional components used to operate an ICD. A battery 10 is used to provide power to charge a capacitor 25 and operate electronic circuitry 15. Electronics circuitry 15 monitor and analyze the cardiac signals detected by the sensing and pacing electrodes 44. When an abnormal condition is detected by the electronic circuitry 15, the capacitor 25 discharges to the therapy electrodes 42 to apply a therapeutic shock to the heart 1. Thereafter, the battery 10 charges the capacitor 25 in preparation for the next shock.
One ongoing challenge is degrading battery performance over time and the eventual need for battery replacement. When the battery is replaced, the entire housing 10 including all ICD components is removed even though only the battery needs service. The capacitors and circuits are not in need of service replacement and are highly reliable components of the ICD system. One shortcoming of the prior art is that the use of a single housing for both permanent components like capacitors and electronic circuitry with non-permanent components like batteries leads to unnecessary removal of ICD components that could otherwise remain implanted and operational.
Additionally, therapy and sensing electrodes are placed to provide accurate detection of cardiac activity and allow for the delivery of therapy. Ideally, therapy electrodes would discharge primarily around or near the cardiac muscle tissue especially the ventricles. As illustrated in FIG. 1 however, the therapy vector created by the illustrated electrode placement results in more tissue than only cardiac tissue being subjected to the shock. The application of ICD shocks to surrounding tissue, nerves and muscle can be painful for patients.
There is a need therefore for improved ICD systems that address the various shortcomings of existing ICD systems and components.